The present invention relates to an offset lithographic printing press. In particular, it relates to gapless tubular printing blankets.
Conventional offset printing presses typically include a plate cylinder, a blanket cylinder and an impression cylinder supported for rotation in the press. The plate cylinder carries a printing plate having a rigid surface defining an image to be printed. The blanket cylinder carries a printing blanket having a flexible surface which contacts the printing plate at a nip between the plate cylinder and the blanket cylinder. A web or sheet material to be printed moves through a nip between the blanket cylinder and the impression cylinder. Ink is applied to the surface of the printing plate on the plate cylinder. An inked image is picked up by the printing blanket at the nip between the blanket cylinder and the plate cylinder, and is transferred from the printing blanket to the web or sheet at the nip between the blanket cylinder and the impression cylinder. The impression cylinder can be another blanket cylinder for printing on the opposite side of the web or sheet material or simply a support cylinder when printing is desired only on one side of the web or sheet.
Conventional printing blankets are manufactured as a flat sheet. Such a printing blanket is mounted on a blanket cylinder by wrapping the sheet around the blanket cylinder and attaching the opposite ends of the sheet to the blanket cylinder in an axially extending gap in the blanket cylinder. The adjoining opposite ends of the sheet define a gap extending axially along the length of the printing blanket. The gap moves through the nip between the blanket cylinder and the plate cylinder, and also moves through the nip between the blanket cylinder and the impression cylinder, each time the blanket cylinder rotates.
When the leading and trailing edges of the gap in the printing blanket move through the nip between the blanket cylinder and an adjacent plate or impression cylinder, pressure between the blanket cylinder and the adjacent cylinder is relieved and established, respectively. The repeated relieving and establishing of pressure at the gap causes vibrations and shock loads in the cylinder and throughout the printing press. Such vibrations and shock loads detrimentally affect print quality. For example, at the time that the gap relieves and establishes pressure at the nip between the blanket cylinder and the plate cylinder, printing may be taking place on the web or sheet moving through the nip between the blanket cylinder and the impression cylinder. Any movement of the blanket cylinder or the printing blanket caused by the relieving and establishing of pressure at that time can smear the image which is transferred from the printing blanket to the web. Likewise, when the gap in the printing blanket moves through the nip between the blanket cylinder and the impression cylinder, an image being picked up from the printing plate by the printing blanket at the other nip can be smeared. The vibrations and shock load caused by the gap in the printing blanket has resulted in an undesirably low limit to the speed at which printing presses can be run while maintaining acceptable print quality.
Conventional printing plates are also manufactured as flat sheets and are mounted in the same way as the printing blankets. The printing cylinders to which the printing plates are mounted also have axially extending gaps in which opposite ends of the printing plates are secured. The adjoining opposite ends of the printing plate also define a gap extending axially along the length of the printing plate.
Smearing of the ink pattern is also promoted by slippage between the surfaces at the nip where the ink pattern is transferred to the printing blanket. Thus, if the speed of the printing blanket surface is either greater or less than the speed of the surface transferring the ink pattern to the printing blanket the surfaces will slip relative to each other which smears the ink pattern.
Several devices have attempted to solve the vibration problem. One such device is disclosed in U.S. Pat. No. 4,913,048. This device attempts to solve the problem by replacing the conventional flat printing plate with a printing plate that is tubular. With this arrangement the tubular printing plate is axially inserted onto and removed from the plate cylinder rather than wrapped around the printing cylinder. With such a device the printing cylinder must be recalibrated both rotationally and axially to take into account the gap extending axially along the length of the printing blanket so that the entire image is printed. Additionally, in a multi-color printing press the printing plate must also be recalibrated relative to the other printing and blanket cylinders. This calibration process takes considerable downtime during which the printing press is not operating. Moreover, since the printing blanket in this device has an axially extending gap vibrations are not eliminated because pressure variations continue to occur both at the nip between the printing cylinder and the blanket cylinder and at the nip between the blanket cylinder and the impression cylinder.
The device disclosed in European Pat. No. 0 225 509 A2 also seeks to reduce vibrations in printing presses. It is similar to the device disclosed in U.S. Pat. No. 4,913,048 except that the printing blanket is also tubular in shape. However, with this arrangement, like the device disclosed in U.S. Pat. No. 4,913,048, every time a printing from needs to be removed, one end of the printing cylinder must be decoupled from the frame. This requires not only removing a portion of the frame, but also extensive adjustments associated with recoupling and realigning the printing cylinder to the frame. This becomes a time consuming task especially since printing forms and plates are generally removed more frequently than printing blankets and they need to be readjusted every time they are removed. Moreover, this device requires considerable modification to the conventional printing press because not only does the frame and blanket cylinder need to be redesigned, but the printing cylinder also needs to be redesigned. Therefore, this device is undesirable because it causes considerable downtime in the printing press and requires expensive modifications to conventional printing presses.
It is an object of this invention to provide an offset lithographic printing press including a gap-free printing blanket which reduces vibrations occurring at high operating speeds in a simple, cost efficient way which avoids considerable downtime in the printing press and involves minimal modification to conventional press design.
An advantage of the present invention is that a gapless printing blanket provides smooth and vibration free rolling engagement between the printing blanket and the printing plate and between the printing blanket and an impression cylinder. This promotes transfer of inked images to the web or sheet without smearing. A further advantage of the present invention is that it obtains these results without having to make significant modifications to the conventional printing press and without having to make complicated readjustments and realignments to the plate cylinder every time a printing plate is changed.
The present invention provides an offset lithographic printing press, comprising: a plate cylinder having an axially extending gap therein; a blanket cylinder engagable with the plate cylinder; and a removable printing blanket mounted on the blanket cylinder, the printing blanket being tubular in shape and having a continuous outer circumferential gap-free surface.
Additionally, the present invention provides a frame which supports the plate and blanket cylinders. A portion of the frame adjacent one axial end of the blanket cylinder is adapted to be moved out of the way in order to provide access to one end of the blanket cylinder to enable a printing blanket to be moved axially onto and off of the blanket cylinder. The tubular printing blanket may be moved axially through the opening in the frame created by movement of the frame portion out of the way.
The present invention also provides means for expanding the printing blanket so that it can be placed on the blanket cylinder, e.g., the cylinder interior may have air pressure applied thereto and passages for communicating air to the outer peripheral surface of the blanket cylinder. Air pressure applied to the interior of the blanket cylinder is thus communicated to the interior of the printing blanket to expand same as it is inserted onto the blanket cylinder. After the printing blanket is located on the outer periphery of the blanket cylinder, the air pressure may be removed. The printing blanket then contracts around the blanket cylinder and tightly engages and grips the cylinder periphery throughout the axial extent of the printing blanket and throughout the circumferential extent of the inner surface of the printing blanket. This pressure relationship between the printing blanket and the blanket cylinder can be relieved by again applying air pressure to the interior of the blanket cylinder to enable the printing blanket to be manually moved off the cylinder.
The present invention further provides that the printing blanket is at least partially formed of a compressible material which is compressed by the plate cylinder at a nip formed between the printing cylinder and the blanket cylinder. By compressing the compressible material at the nip, the outer surface of the printing blanket has a surface speed which is substantially the same at locations immediately before the nip, at the nip, and immediately after the nip. This prevents slippage between the surfaces of the printing plate and printing blanket before, at, and after the nip to prevent smearing of the ink pattern.
The tubular printing blanket has a cylindrical outer layer of incompressible material and a cylindrical layer of compressible material on an inner layer of rigid material. The outer layer of the printing blanket is deflectable to compress the compressible layer of the printing blanket. The compressible layer of the printing blanket contains a plurality of voids which are relatively large before the compressible layer is compressed and which are relatively small in the portion of the compressible layer which is compressed by deflection of the outer layer of the printing blanket at the nip.
The rigid inner layer of material is stressed in tension by the blanket cylinder to provide a tight pressure relationship between the printing blanket and the blanket cylinder. This pressure relationship fixes the printing blanket on the blanket cylinder so that there is no relative movement therebetween during operation of the press. The press includes means for effecting radial expansion of the tubular printing blanket while on the blanket cylinder to relieve the pressure relationship between the printing blanket and blanket cylinder. When the pressure relationship is relieved, the printing blanket may be manually moved axially off of the blanket cylinder. Also, the printing blanket must be expanded radially (tensioned radially) outwardly in order to permit movement of the printing blanket axially onto the blanket cylinder. The press is also provided with structure for performing this function.